EVERYBODY was so stoned at Woodstock — 40 years ago this August — that a definitive book on the legendary rockfest can probably never be written, according to Pete Fornatale. In “Back to the Garden: The Story of Woodstock,” out next week, the veteran rock deejay attempts to separate fact from fiction, but notes, “Woodstock is an elephant. Perhaps even a big pink one, depending on what you were ingesting back then.” He adds that “400,000 versions of the truth” from those who were there — mixed with those who swear they were there but weren’t, and those who know it from records, books and movies — shows “how chameleon-like anything Woodstock-related is.”